linux-kernel - Re: [PATCH net-next v4] net: sfp: add SMBus I2C block support

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Message-ID: <397e0cdd-86de-4978-a068-da8237b6e247@gmail.com>
Date: Fri, 9 Jan 2026 17:48:27 +0100
From: Jonas Jelonek <jelonek.jonas@...il.com>
To: Maxime Chevallier <maxime.chevallier@...tlin.com>,
 Russell King <linux@...linux.org.uk>, Andrew Lunn <andrew@...n.ch>,
 Heiner Kallweit <hkallweit1@...il.com>,
 "David S . Miller" <davem@...emloft.net>, Eric Dumazet
 <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>,
 Paolo Abeni <pabeni@...hat.com>
Cc: netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
 Bjørn Mork <bjorn@...k.no>
Subject: Re: [PATCH net-next v4] net: sfp: add SMBus I2C block support

Hi Maxime,

On 09.01.26 16:51, Maxime Chevallier wrote:
> Hi,
>
> On 09/01/2026 11:13, Jonas Jelonek wrote:
>> Commit 7662abf4db94 ("net: phy: sfp: Add support for SMBus module access")
>> added support for SMBus-only controllers for module access. However,
>> this is restricted to single-byte accesses and has the implication that
>> hwmon is disabled (due to missing atomicity of 16-bit accesses) and
>> warnings are printed.
>>
>> There are probably a lot of SMBus-only I2C controllers out in the wild
>> which support block reads. Right now, they don't work with SFP modules.
>> This applies - amongst others - to I2C/SMBus-only controllers in Realtek
>> longan and mango SoCs.
>>
>> Downstream in OpenWrt, a patch similar to the abovementioned patch is
>> used for current LTS kernel 6.12. However, this uses byte-access for all
>> kinds of access and thus disregards the atomicity for wider access.
>>
>> Introduce read/write SMBus I2C block operations to support SMBus-only
>> controllers with appropriate support for block read/write. Those
>> operations are used for all accesses if supported, otherwise the
>> single-byte operations will be used. With block reads, atomicity for
>> 16-bit reads as required by hwmon is preserved and thus, hwmon can be
>> used.
>>
>> The implementation requires the I2C_FUNC_SMBUS_I2C_BLOCK to be
>> supported as it relies on reading a pre-defined amount of bytes.
>> This isn't intended by the official SMBus Block Read but supported by
>> several I2C controllers/drivers.
>>
>> Support for word access is not implemented due to issues regarding
>> endianness.
> This patch should probably be accompanied with a similar addition to the
> mdio-i2c driver. for now, we only support full-featured I2C adapters, or
> single-byte smbus, nothing in-between :(
>
> Do you have something like this in the pipe ? not that this blocks this
> particular patch, however I think that Russell's suggestion of making
> this generic is the way to go.

I agree to Russell's suggestion and will work on that. Apart from that, I haven't
considered a similar change for mdio-i2c yet. No issue to deal with that but 
I think I have no way to test this properly.

> Maxime

Best,
Jonas